Solved. I dont' know exactly what was wrong, although it was most probably in the callback function. Maybe the carriage returns.
But this works fine:
$.ajax({
async: false,
url: cart_doc,
type: 'POST',
data: {action: 'retrieve_opt', name: fieldName},
dataType: 'text',
timeout: 1000,
error: function(){
alert('Error retrieving product option value');
},
success: function(chosenOption) {
field.value=chosenOption;
alert(field.value);
}
});
Bye.

